I get my single coil cartos directly from Smoktech:
Single Coil XL Tank Cartomizer for the SmokTank XL
You can order
one or two hole, pre-drilled cartos in 1.7, 2.0, or 3.o oHm resistance for $1.99 each. I use them on my Silver Bullet and BB in several Smoktech DCT tanks.
I just recently switched most of my tanks from dual coil to single coils. I had read some discussion in a couple of threads here that the dual coils don't really fire effeciently unless given around 4.8v power. Given my devices are all 3.7v, I decided to give them a try. I got several in each, one and two hole, at 2.0 oHm. Plus, I read that dual coils are harder on batteries, and work best with the high drain batteries.
I think I like them better than the dual coils. So far I haven't gotten any burnt taste like I occassionally got with the dual coils. The vapor and flavor are similar if not better. I choose the number of holes to fit the viscosity of the juice I'm using.
So far, the only juice that has had wicking issues is my Boba's, which is an extremely thick juice (all vg if I'm not mistaken). I've even attempted to dilute it with some water, but it is still a stiff pull and I find I need to top off the carto frequently.